Conventionally, there is a clamp-on ultrasonic flow-meter clamped onto a pipe in which fluid flows and of which the flow-rate is measured from outside the pipe, which is described in e.g. Patent Documents 1 to 3 or the like. Of the above ultrasonic flow-meter, a pair of ultrasonic-sensors is provided thereon by which ultrasonic-waves are obliquely propagated into the fluid flowing within the pipe. The transmission and reception of said ultrasonic-waves is switched between the pair of ultrasonic-sensors for measuring the time lag of said ultrasonic-waves propagating into the fluid, thus calculating the flow-rate of the fluid.
The clamp-on ultrasonic flow-meter, as described in Patent Documents 1 and 2, employs a method for detecting the reflected ultrasonic-waves by providing a pair of ultrasonic-sensors on the same side of the pipe onto which the flow-meter is clamped. The clamp-on ultrasonic flow-meter, as described in Patent Document 3, employs a method for detecting the transparent ultrasonic-waves by providing a pair of ultrasonic-sensors on the opposite side of the pipe onto which the flow-meter is clamped.